GB2298396A

GB2298396A - Fluid clutch for web media transport in an image printer

Info

Publication number: GB2298396A
Application number: GB9604230A
Authority: GB
Inventors: Werner Fassler; James Edward Pickering; John Edward Mooney
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1995-03-01
Filing date: 1996-02-28
Publication date: 1996-09-04
Anticipated expiration: 2016-02-28
Also published as: GB2298396B; JPH08270685A; DE19607094A1; GB9604230D0

Description

2298396 FLUID CLUTCH FOR WEB MEDIA TRANSPORT IN AN IMAGE WRITER

BACKGROUND OF THE INVENTION

Technical Field

This invention relates generally to media transport systems for image writers such as copiers and/or printers, and more particularly transport systems for moving web medium past a print head. The invention is particularly useful in a transport system for moving dye-donor media past a thermal print head to transfer a dye image to a receiver medium, but other applications for the invention will readily occur to those skilled in the art.

Background Art

Referring to Figure 1, a thermal printer 10 according to a preferred embodiment of the present invention includes a print head assembly 12 and dye donor web supply and take-up spools 14 and 16, respectively. The printer includes a roller platen assembly 18, a pair of dye receiver medium transport mechanism pinch rollers 20 and 22, and a dye receiver medium supply 24.

Printer operation begins with a loading phase, in which print head assembly 12 moves to a loading position. A dye-donor web 26 and a sheet 28 of dye receiver medium advance along converging paths to a printing location. As a sheet 28 of dye receiver medium advances, it moves along a guide 30 to follow a curved path toward a gap between print head assembly 12 and platen assembly 18. As the dye receiver medium moves into this gap, it contacts dyedonor web 26 and is guided toward dye receiver medium transport mechanism pinch rollers 20 and 22.

Dye-donor web 26 is pulled from supply spool 14 one print line at a time. To prevent uncontrolled unwinding of the dye-donor web from the spool, it is common to apply a slight drag force to the spool. However, standard clutches exhibit a large stgtic frictional force which must be overcome when a load or torque is first applied to them. This initial large friction force is sometimes referred to as Nstiction. Once the initial stiction is overcome, the force needed to operate the clutch is greatly reduced to sliding friction values, as illustrated in Figure 2.

The need to overcome initial stiction can produce media transport motions which are jerky and non consistent, resulting in image banding artifacts. Accordingly, it is very desirable to have a media transport mechanism that does not exhibit the jerkiness associated with standard clutches.

The stiction problem has been overcome in the past by constantly driving the supply drag clutch in the reverse, or take-up, direction so that there is always relative movement between the two clutch halves. Thus, there is never the need to overcome static friction.

However, this requires a clutch drive motor, and adds greatly to the cost, complexity, and maintenance requirements of the image writer.

DISCLOSURE OF THE INVENTION

According to a feature of the present invention, an image writer includes a print head, a media supply adapted to receive a roll of web material, a transport system for moving the web in a forward direction from the media supply past the print head, and a drag clutch associated with the media supply for applying tension to the web opposing the web drive. The drag clutch has first and second opposed surfaces that are separated by a viscous fluid which shears to create a drag force resisting relative movement between the opposed surfaces without exhibiting more than negligible stiction.

-3 is BRIEF DESCRIPTION OF THE DRAWINGS in the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:

Figure 1 is a schematic of a prior art thermal printer which can be employed to make color images in a dye receiver medium;

Figure 2 is a schematic illustration of the forces involved in stiction; Figure 3 is a schematic illustration of the forces involved in the fluid clutch according to the present invention; Figure 4 is a sectional view of a fluid clutch according to the present invention; and Figure 5 is an end view of the clutch shown in Figure 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Figure 3 is a schematic view of the desirable forces on a media transport system that would provide a smooth transition from a no-load condition to a loaded condition without significant stiction problem. Such a transport system would provide smoother media transport, more accurate media positioning, improved print quality, and possibly quieter operation.

Referring to Figures 4 and 5, a non-stick viscosity drag clutch includes an input shaft and gear 32 and 34, respectively. Gear 34 can, of course, be a belt pulley, a slip drive, etc. A casing 36 is attached to gear 34 and turns therewith.

A plurality of large diameter disks 38 turn with gear 34 and casing 36, perhaps by a disk drive dog 40 (Figure 5). A smaller-diameter disk 42 is positioned between each pair of large-diameter disks 38 to turn with an output shaft 44, such as by way of engagement with a flat on shaft 44 as shown in Figure 5. A cover 46 retains the disks in casing 36, and the disks are pressed against each other by a retainer ring 48.

Casing 36 is filled with a none drying grease through which rotation of shaft 32 is transmitted to shaft 44 via disks 38 and 42. The viscosity clutch operates by shearing the wet, sticky grease. At low speeds, there is a small amount of shear loss resulting in low drag values. Drag increases with speed of the output. Drag is also a function of surface area where shearing occurs. The clutch can be operated at a desired drag for a given velocity by proper selection of the surface area (disk diameter and number of disks), the viscosity of the grease, and the pressure applied by, retaining ring 48.

What io claimed ia:

1. An image writer having a print head, a media supply adapted to receive a roll of web material and a transport system for moving the web in a forward direction from the supply past the print head; characterized by a drag clutch associated with the media web supply for applying tension to the web opposing the transport system, said drag clutch having first and second opposed surfaces that are separated by a viscous fluid which shears to create a drag force resisting relative movement between the opposed surfaces without exhibiting more than negligible stiction.

Claims

2. An image writer according to Claim 1 wherein the print head is a

thermal print head; and the web material is dye donor web material.